Magnetoresistive Random Access Memory (MRAM) is a non-volatile computer memory technology that has been under development for several years. Unlike conventional RAM chip technologies, in MRAM data is typically stored by magnetic storage elements. Such elements may be formed from two ferromagnetic plates, each of which can hold a magnetic field, separated by a thin insulating layer. One of the two plates may be a permanent magnet set to a particular polarity. The field of the other plate may be changed to match that of an external field to store memory. This configuration is known as a spin valve. A memory device may be built, for example, from a grid of such “cells”.
Reading of MRAM data may be accomplished by measuring the electrical resistance of the storage element. A particular element may be selected, for example, by powering an associated transistor which switches current from a supply line through the element to ground. Due to the magnetic tunnel effect, the electrical resistance of the cell changes due to the orientation of the fields in the two plates.
By measuring the resulting current, the resistance inside any particular element may be determined, and, from this, the polarity of the writable plate. For example, if the two plates have the same polarity this may be considered to mean “1”, while if the two plates are of opposite polarity, the resistance will be higher, and may be interpreted as “0”.
Data may be written to the elements using a pair of write lines arranged at right angles to each other, above and below the element. When current is passed through the lines, an induced magnetic field may be created at the junction, which the writable plate may pick up.
In general, a magnetic tunnel junction (MTJ) consists of two layers of magnetic material separated by an insulator. If a voltage is applied across the two materials, a current may be induced to flow through the magnetic tunnel junction. This current will differ based on whether the magnetizations of the two magnetic materials are parallel or antiparallel.
As described above in the context of MRAM, a single bit of data may be stored in an MTJ in this manner. For example, the MTJ can be deemed to store the value of “1” if the magnetizations are antiparallel and “0” if the magnetizations are parallel. The bit can be read by applying a voltage to the MTJ and measuring the current passing through. When an MTJ is used to store data as MRAM, one of the layers may be deemed as the fixed layer, with static magnetization, and one of the layers may be deemed as the free layer, in which the direction of the magnetization is changed based on whether to store a “1” bit or a “0” bit.
The magnetization of the free layer of the MTJ may be changed using magnetic field writing. Despite ongoing research in the field, it has proven difficult to produce MRAM chips based on a transistor size less than 65nm when using magnetic field writing to change the magnetization of the free layer.